I'll give it a try, but I'm much better in person. :)
First decide if you want a denser shorter shrub or whether you want to go with a more open framework. Your rhody is naturally going for the more open framework, a look which this kind of rhody lends itself to. These actually grow into small trees. They look great underplanted with perennials and go well in a partly shaded position. Too much sun may scorch the leaves.
When I prune for an open framework, first I look at the major branches of the shrub and decide what the basic shape is going to be. Yours looks like it would make a nice open vase shape, kind of like the letter 'U'. If you go for this, first take off any broken or damaged branches, any branches that are not growing in the right direction, or any branches that are rubbing against other ones. Cut these back all the way to a main branch as close as you can get. Any little 'nubs' left may sprout leaves. Then look at what you have left and decide which of the main branches you want for the main framework of the shrub. Yours looks like it has about 4 main trunks, but one of them appears to be cut off at the top. I can't really tell for sure in the photo. If it is, I would consider taking that branch off all the way to the ground, or to the main trunk. It's going to start putting out leaves all down because the 'leader' has been removed. This causes the plant to release growth hormones at the growth points along the branch. You could leave the secondary branches that go off to each side.
Take a look at the bottom left hand branch. There are two options here. That branch looks like it's growing a bit strangely close to the trunk, like there is a crooked part. If it is, you can take the entire branch off. If it's okay or the crook doesn't look bad, then you can start opening up the framework by taking some of the secondary branches off. These would be the ones that are growing straight up from a major branch and block the view of a nice clean framework. If I were a computer-wise person, I would likely be able to mark the ones for you. Basically, just take off anything that doesn't work with the design you have in mind. Take it all the way to where it starts growing. These are called 'thinning' cuts and will not cause the shrub to get bushy. If you just lop off the end of a branch, called a 'heading' cut, that will stimulate the shrub to put out leaves all along the branch, as I explained above. That's not a good look. Try not to take more than a third of the growth off of the shrub.
Now if you want to have a shrubbier look, just do 'heading cuts' on the major branches after you've taken off any spindly, broken, etc. branches. This will also stimulate the shrub to start putting out new growth from the roots. Once this rhody is established you could even cut it all the way to the ground and it would grow back beautifully.
I have a white rhody that I didn't even know existed when I moved into this house. I guess it had been cut down to the ground. The year after we moved in, I started seeing growth emerge and I had a nice shrubby shrub. The following year it bloomed. We've been here 5 years and now it's time for me to go for the pruners and give it some shape.
Here's a tip I forgot to put in. When pruning for an open framework, I like to do what I call 'inside out' pruning. Basically, I get underneath the shrub or tree and look up into the interior of it. I sometimes will prune from this perspective, always taking time to stand back and look at it from the outside, too. If you are going for an open, airy look, this is a good way to do it.
Hope that helps!
